A detailed description of prior art bus voltage limiters (BVL) for spacecraft solar arrays is disclosed in Ahrens et. al. U.S. Pat. No. 4,691,159, assigned to the assignee of the present invention and incorporated herein by reference. As discussed in Ahrens, conventional power regulating systems for spacecraft solar panels use shunt dissipative voltage limiters (see FIGS. 4 and 5 of Ahrens) or boost switching limiters (see FIGS. 6-9). Both use a pulse width modulation scheme to maintain a substantially constant output voltage. While the dissipative limiters dump overvoltage through a switch in shunt with the array, the boost regulator controls the voltage produced across an inductor connected in series between the solar array and the load. The voltage supplied to the load is the sum of the supply voltage and the inductor voltage. The pulse width modulator, by controlling the voltage boost provided by the inductor, regulates the output voltage. There are tradeoffs in the two approaches including the high localized heating associates with the dissipative limiters and the end-of-life power waste associated with boost limiters. The Ahrens patent is directed toward resolving these conflicting constraints by providing a regulating system in which the solar panel array is divided into a constant current part and a constant voltage part with a boost switching regulator connected to receive power from only the constant current part.
Another prior art solar panel regulator is the sequential full shunt limiter shown in FIG. 1. The solar panel control module includes a PWM controller, four shorting switches, four isolation rectifiers, a local filter capacitor C1 and a low pass filter L1C2. A complete solar panel regulator system would includes several of these modules connected together at the input of the low pass filter, each having its DC output voltage adjusted to regulate at a slightly different voltage. Each regulator then has three operating modes determined by the bus voltage (and load): all switches off, modulated drive to the switches, or all switches on. This regulator controls up to 2.5 KW of solar array power to provide a regulated output bus at 51.75.+-.0.75 Vdc. However, it is not without shortcomings, such as the emission of relatively high levels of electromagnetic interference (EMI). This is due to the method of solar array shunting that generates 50 volt square-waves at high frequency (40 Khz) which are present on portions of the solar array wiring external to the spacecraft. Filtering of the lines is difficult because it imparts higher electrical stresses on the shunting elements (i.e., power MOSFETs) in the BVL. More effective filtering can be implemented, where electro-magnetically sensitive programs are involved, but at a significant weight penalty. In addition to emitting high levels of EMI, the prior art BVL designs are relatively heavy and have proven to be somewhat difficult to produce. The pulse width modulation approach selected in the prior art limiter of FIG. 1 is difficult to analyze for control loop stability. During manufacture and test, the circuitry is highly sensitive to internal wiring layout and component placement. This appears to be due at least in part to the high levels of EMI that are present on the solar panel wiring inside the unit.
A coupled inductor type boost DC/DC converter power stage has been employed in other applications such as the battery discharge controller disclosed in U.S. Pat. No. 5,122,728, incorporated herein by reference. In that application the converter operates from a low impedance power source (i.e., the spacecraft battery) and supplies power to the regulated spacecraft bus. The control laws which govern that application require that the switching converter increase the duty cycle in response to an increase in load current or power. The power source voltage remains essentially constant over wide variations in load power while power source current experiences wide variations. In the bus voltage limiter application of the present invention the duty cycle decreases in response to a load increase, and the input power source voltage varies significantly over variations in load power while the input current remains essentially constant due to the nature of the photovoltaic solar array source.